In recent years, many containers such as PET bottles and metal cans are covered with heat-shrinkable labels prepared by performing printing on a base film of a heat-shrinkable resin film.
As such a heat-shrinkable resin film, films comprising polystyrene type resin predominate since it is superior in low temperature shrinkability. However, since the polystyrene type resin film has inadequate heat resistance, there was a problem that for example, the heat-shrinkable resin label may be shrunk and the label may be distorted or broken when a PET bottle topples in being heated in a hot warmer equipped in a convenience store, and the like. Furthermore, the polystyrene type resin film also has a problem that since it has inadequate solvent resistance, it may be shrunk or dissolved by the adhesion of oils when it is used for the packaging of articles containing oils.
On the other hand, efforts to use a polyester type film, which is superior in heat resistance and solvent resistance, in place of a polystyrene type resin film as a heat-shrinkable label are made. But, the polyester type film has a problem that it tends to produce crinkles when it covers a container since it has bad low temperature shrinkability and is shrunk rapidly. Further, perforation for tearing off is often provided on the shrink film so that the heat-shrinkable label can be easily torn off and removed from the used container in order to recycle containers, but in the polyester type film, there was also a problem that tearing properties along the perforation are poor, and therefore the heat-shrinkable label cannot be easily torn off and removed from the container. Furthermore, in the polyester type film, there was a problem that since it has large shrinking stress, in the case of using the polyester type film as a label for hot beverages, the label shrinks and compresses the container due to heating of the beverages at the time of sale and thereby a level of the content of the container rises to cause the content to spill over the container.
For this situation, in Patent Document 1, a hard multilayer shrinkable film obtainable by laminating outer surface layers comprising a polyester type resin on an intermediate layer comprising a polystyrene type resin with an adhesive layer comprising an olefin type resin interposed between the outer surface layer and the intermediate layer, is disclosed. Further, in Patent Document 2, a heat-shrinkable label, which includes a base film obtainable by laminating outer surface layers comprising a polyester type resin comprising a specific monomer on both sides of an intermediate layer comprising a polystyrene type resin, and by laminating the outer surface layers and the intermediate layer without an adhesive layer interposed between the outer surface layer and the intermediate layer, is disclosed. Furthermore, in Patent Document 3, a laminated film having a surface layer comprising a polyester type resin, an intermediate layer comprising a styrene type resin and an adhesive layer comprising an adhesive resin, is disclosed. The heat-shrinkable labels comprising these multilayer films are superior in low temperature shrinkability and tearing properties along the perforation by the virtue of the intermediate layer comprising a polystyrene type resin, and further they are also superior in solvent resistance and heat resistance since the intermediate layer is covered with the outer surface layers comprising a polyester type resin.
However, if these heat-shrinkable labels cover the containers actually, there were problems that in the hard multilayer shrinkable film described in Patent Document 1, the intermediate layer may be peeled off from the outer surface layer in covering the container, and in the heat-shrinkable label described in Patent Document 2, after covering the container with the heat-shrinkable label, if the films rub against each other during transportation of products or the film is scratched with a fingernail or a body, peeling may occur between an intermediate layer and an outer surface layer.
Further, when the heat-shrinkable labels are used for PET bottles for drinking, decoration of labels is performed by printing for the purpose of differentiating products from those of other companies or enhancing images of user. However, since printing ink used in a printing step of the label generally contains an organic solvent, a trace of the organic solvent remains on the printed surface after printing and drying. Thereby, a problem that when the heat-shrinkable film described in Patent Document 3 is used, an adhesive property between the outer surface layer and the intermediate layer is affected by a residual organic solvent and adhesion strength between the outer surface layer and the intermediate layer after a printing step is significantly reduced in comparison with that before the printing step has newly arisen.
Accordingly, a heat-shrinkable multilayer film, which, in the case of using the heat-shrinkable multilayer film as a heat-shrinkable label for a container, does not produce peel between the outer surface layer and the intermediate layer in covering a container, is superior in heat resistance, oil resistance, tearing properties along the perforation and appearance, is less-affected by an organic solvent used in a printing step and has adequate adhesion strength after the printing step, has been required.    Patent Document 1: Japanese Kokai Publication Sho-61-41543    Patent Document 2: Japanese Kokai Publication 2002-351332    Patent Document 3: Japanese Kokai Publication 2006-15745